Heat-sensitive retroreflecting materials

ABSTRACT

An article useful as a label or thermal telltale which becomes visibly retroreflective in response to simple heating, provided by retroreflective elements masked with a heat-sensitive lightobscuring layer which is removed from the retroreflective elements by heating. A liquid-coating composition is also provided for preparing the article.

111111100 Smiles 1801101111 (Imarneya 51 Mar. 1141, 11972 [54]H-IllEAT-SENSETHVE RETROREFLECTIING 2,574,971 11/1951 Heltzer ..350/105X MATERI LS 2,963,378 12/1960 Palmguist et al .106/193 R 2,997,4038/1961 Searight 350/105 X 1 Inventori mice Cwmeya, Alexandrla, Minn-3,187,068 6/1965 De Vuer et a1. ..261/1 3 441948 4/1969 Ondis......l17/36.7 X 3 I t M 11 M f 1 [7 1 Asslgnee 2: 53:23; SL l sifi f ac3,511,700 5/1970 Miro ..117/36.7 x 1 Filedi J y 1970 PrimaryExaminer-Alfred L. Leavitt Assistant ExaminerWm. E. Ball 21 A l.N 53,3861 pp 0 Attorney-Jimmy, Alexander, Sell, Steldt & Delahunt [52] US. Cl...117/2011, 117/35 R, 73/356, [57] ws R C 73/358, 260/37 R, 260/41 R,106/193 R, 106/241,

264/] An article useful as a label or thermal telltale wh|ch becomes[51] 1111. C1 ..EMc 1/18 visibly retmreflective in response to Simpleheating Frwided [58] m of Search 17/35 R 35 V, 35 s 201 byretroreflective elements masked with a heat-sensitive light- 1 17/212,232, 36.7, 36.8; 260/37 R, 41 R; 106/193 obscuring layer which isremoved from the retroreflective ele- R 241 2 7; 2 4/ 350 105; 73/35 35ments by heating, A liquid-coating composition is also provided forpreparing the article. 36] Regmms med 8 Claims, 4 Drawing Figures UNITEDSTATES PATENTS 2,407,680 9/1946 Palmquist et a1. ..350/105 PATENTEDMAR14 I972 3, 649,352

INVENTUR CAL/CE G. COUPNEYA A 1- TORNE vs HEAT-SENSITIVE RETROREFLECTINGMATERIALS This invention relates to articles having a surface coating orlayer which is capable of being rendered retroreflective by simpleheating, and which are therefore useful as labels, thermal telltales,and in other ways. The invention also relates to a coating compositionfor making the articles. One particularly important application is inthe detection, in electronic circuits, of defective components which arefrequently difficulty accessible for testing. Such elements are easilyprovided, either before or after installation ponent is then quickly andeasily located by retroreflection of light rays from a test lamp.

Products of the invention may be in the form of thin sheet materialswhich may be used as such, e.g., in the preparation of labels, or may beapplied to various articles as telltales or the like. The invention alsocontemplates forming the inherently retroreflective coating directlyupon the surface of an article, by application of liquid-coatingcomposition which is then dried.

Retroreflective coatings have previously been described. A well-knownform employing tiny glass beads on a reflective support and covered witha transparent covering having a flat face is shown in U.S. Pat. No.2,407,680. US. Pat. No. 2,574,971 describes the application of coatingscontaining beads in a nontransparent paint which at first obscures, butby attrition is subsequently removed from, the upper surfaces of thebeads. Light-sensitive and chemically reactive coatings have beenapplied over retroreflective layers, e.g., as described in US. Pat. No.3,222,986.

The present invention likewise may employ transparent beads as theretroreflective component, while in addition ineluding means forobscuring or masking the retroreflective elements which means can beremoved from the retroreflective elements by heating.

in US. Pat. No. 2,668,126. tive, and may indeed be light-diffusing.

As is well known, the characteristic of certain reflectors in returningback a brilliant cone oflight toward the source ofan angularly incidentlight beam is generally designated by the term reflex reflection." Thistype of reflection is to be distinguished from the reflection of mirrorswhich is specular reflection, and from the action of diffusing types ofreflective surfaces which dissipate the incident light in all directionswithout selective return in the direction ofincidence. Areflexreflecting element may take the form of a transparent spherehaving a forward lens surface for refracting incident light, togetherwith a rearward reflector for returning the light through the lenssurface. The retroreflecting elements are generally so small they arepractically unnoticeable when viewed directly but are neverthelesscapable of powerful retroreflective action when appropriately positionedwith respect to an illumination source.

in the present invention the retroreflective elements are masked by anobscuring material which has an initial overall nontransparent, e.g.,opaque or translucent, state that will not permit light rays to pass ina coherent pattern. The light rays impinging upon the surface of a layerof the material will either be absorbed, diffused in all directions, orotherwise reflected therefrom. The obscuring material, normally in asolid state, achieves a fluid state and will flow to physically expose aportion of the surface of the retro-reflecting elements when heated atthe requisite temperature, thereby permitting light rays to beretroreflected.

To better understand the present invention, reference is directed to theaccompanying drawings wherein:

FIGS. El through 3, inclusive, are representations in cross sectionofvarious aspects ofthe present invention; and

FIG. 4 is a view in perspective of an electronic component having a bandthereon formed according to the present inventron.

In FIG. 3, article 10 comprises a vast number of minute lightretroreflective particles 11 imbedded in heat-sensitive obscuring layer12 which is capable of preventing the passage of a coherent pattern ofvisible light therethrough. This light dissipation is graphicallyrepresented in FIG. 1 by light rays (a). Heating article It) at therequisite temperature will cause the obscuring layer 12 to be convertedfrom a normally solid state to a fluid state and to flow away from thebeads by filling the voids within the blush layer thereby exposing thetops of the retroreflective particles ll, providing retroreflection oflight.

Retroreflective particles ll shown in article 10 of FIG. II areinherently retroreflective and heat resistant, i.e., capable ofwithstanding the temperatures required to convert the obscuring layer tothe fluid state. Specifically useful retroreflective particles meetingthese requirements are half-reflectorized transparent beads such asthose disclosed in the teachings of U.S. Pat. No. 2,963,378. Vastnumbers of these minute beads are randomly dispersed in obscuring layer12 of article It). The beads are inconspicuous until obscuring coating112 has been subjected to the requisite heating. Once the coating hasbeen heated to the requisite temperature the beads are stillindividually inconspicuous but an over-all visual effect is realizedwhen a beam of light rays is directed toward the coating. A brilliantreflection of light is had in the heated areas of the coating, providingan observer with an enhanced visual stimulus as to the informationcontained on the label. After being subjected to the requisite heating,the retroreflective elements are permanently partially exposed and thearticle becomes retroreflective.

FIG. 2 shows the article 10a after having been subjected to therequisite temperature. The heated area is characterized by havingprotruding retroreflective elements 11 embedded in layer 12a derivedfrom the obscuring layer. Retroreflection is graphically represented inFIG. 2 by light rays (b).

FIG. 3 shows an article 15 which has been partially subjected to therequisite heating. Heated area 16 is characterized by havingretroreflective elements protruding therefrom, permittingretroreflection, whereas the unheated portion of the article 115 willnot retroreflect light.

Various other layers may be contained in the article depending upon theparticular requirements. For example, a flexible sheet materialrepresented by layer 17 in FIG. 3 can be provided as a supportingcarrier, and an adhesive layer 18 can be provided on the lower surfaceto facilitate attachment of an article sheet to a substrate.

One important application of the aforementioned features of the presentinvention is in the area of electronics. The invention finds use indetecting defective electronic components which generally show noexternal signs of failure occasioned by excessive loading resulting inoverheating. FIG. 4 shows one such component, e.g., a resistor 19,having a band 20 thereon formed in accordance with the invention. Whenthe resistor heats during failure the band 20 becomes retroreflectiveand easily conspicuous under an inspection light.

The retroreflective elements are generally no more than 0.1 inch inmaximum extent and preferably no more than 0.01 inch. Specificallyuseful retroreflective elements are halfreflectorized glass beads whichhave diameters ranging from [0 to 50 microns and a refractive index ofat least about 1.8, preferably about L9. The beads can be provided witha reflector half cap by various procedures. One way is to use ahighvacuum metal vapor deposition procedure. Aluminum and tin may bedeposited in this way. Another procedure involves forming a releasablemonolayer of beads on a heat-resistant carrier and plating thehemispherical upper surface by exposure to metal carbonyl gas that isreduced in situ. Nickel and chromium coatings can be applied in thisway. Still other procedures involve silver-plating and spraying withvarious materials. Since the above-mentioned procedures are applicationsof well-known processes, detailed working descriptions are not deemednecessary.

The retroreflective elements are imbedded in or otherwise masked byaforementioned obscuring layer. When a light beam is directed toward acoated article made according to the teachings of the invention. nolight can be retroreflected unless the obscuring layer has achieved therequisite tempera ture. Utilizing this capability the obscuring materialprevents light rays from effectively striking the retroreflectiveelements, making retroreflection impossible until the obscuring layerhas been rendered fluid and flows from the top portion of theretroreflective elements. At least about percent of the total surfacearea of a bead must be exposed before adequate retroreflection isachieved.

One type of material having the above property is that categorized asblush lacquer coatings of normally transparent fusible resins.Microscopic examination of blush lacquer films reveals numerousmicroscopic bubbles throughout their entire structures. These very smallhollow spaces give the film a light or opalescent appearance commonlycalled blushing. Blushing may be accomplished by welLknown methods whichinvolve the actual inclusion of water or air in the hollow spaces of thefilm. The preferred blush lacquer coatings of the present in vention arevesicular structures characterized by interconnected hollow spaces whichhave no water inclusions. Specifically useful blush lacquer films may beprepared according to the teachings disclosed in U.S. Pat. No.2,519,660.

Blush lacquer films are rendered transparent and caused to contract to alesser thickness when heated by elimination of the minute hollow spacesthroughout the heated area of the film. Hollow spaces in a blush lacquerfilm made of a thermoplastic fusible resin can be eliminated by heatingthe film to a temperature sufficient to cause the vesicular resinstructure to collapse and coalesce to form a thinner clear film.

A blush lacquer coating may be prepared by dissolving a normallytransparent thermoplastic fusible resin in a solvent having a vaporpressure in excess of that of water and further diluting the resinsolution with water, or a water-solvent mixture. The solvent must be onewhich is miscible with water while the resin must be substantiallyinsoluble in water. If enough water is added to a solution of such aresin, the resin will precipitate; therefore, less water than isrequired to precipitate the resin should be added. This solution iscoated and allowed to dry. Additionally, blush coatings may be preparedusing a mixture of two solvents with the resin, thereby producing ablush film without the use of water.

Blush lacquer films can be prepared from film-forming thermoplasticfusible linear polymer resins. Exemplary resins include ethyl cellulose,cellulose acetate-propionate, cellulose acetatc-butyrate, benzylcellulose, polymethyl methacrylate, and polystyrene.

The temperatures and times required to achieve the fluid state for blushcoatings may be modified by the addition of various agents. For example,the addition of subliming solids such as naphthalene will extend thetime required to achieve the fluid state at the requisite temperature.Additionally, plasticizers lower the fluidizing temperature. Primaryplasticizers are preferred over secondary, mechanical or lubricantplasticizers which may soften the coating. Dioctyl phtbalate will reducethe fluidizing temperature of blush ethyl cellulose. Tricresylphosphate, diamyl phthalate, dicapryl phthalate, dibutoxyethylphthalate, tributyl citrate, butyl phthalyl butyl glycolate, and dibutylsebacate have all been found to reduce the fluidizing temperature ofblush cellulose acetate.

Plasticizers of the type discussed may be added in variable amounts tothe resin, depending upon the desired reduction in fludiizingtemperature. As it can be seen, the fluidizing temperature of theparticular article can be tailored to meet the needs of the user.

Other well-known imaging systems may be adapted to provide the obscuringlayer of the present invention. For example, an obscuring layer mayinclude an obscuring amount of fusible organic particles which melt andcoalesce upon heating to become a thinner permanently transparent layer.

The obscuring material need only be provided in a layer having athickness sufficient to prevent the retroreflection of light, e.g.,cover the tops of retroreflective beads. The obscuring layer, however,must not be so thick as to cover the entire retroreflective elementsafter fluidization and the achievement of a lesser thickness. At least10 percent of the surface area of a transparent bead must be exposedbefore adequate retroreflection oflight will be provided.

A liquid coating composition of the present invention may be prepared bymixing a blush lacquer solution with particles which are adapted toretroreflect light such as the aforementioned half-reflectorized glassbeads. For this usage the coating composition must contain enoughlacquer to obscure the reflective properties of the retroflectiveparticles when the coating has dried. The blush lacquer solutionconsists of a lacquer mixed according to well-known teachings such asthe aforementioned US. Pat. No. 2,519,660, to a paintlike con sistencywhich can be deposited in a film which dries to form a blush layer. Thesolution may contain from 1 to 20 percent or higher of nonvolatilelacquer components, depending upon their solubility and upon the fluidconsistency desired. Upon application to base surfaces, the tiny glassbeads in the coating composition will spread out to form at least amonolayer containing tens of thousands of beads per square centimeter.The dried blush lacquer coating will be ofjust sufficient thickness toobscure the layer of beads.

The coating compositions of the present invention can be utilized in anaerosol bomb or spray-can-type applicator from which the composition canbe sprayed on any desired surface. The pressure is provided by avolatile propellent liquid having a high vapor pressure that is includedin the vessel. Additionally, the coating composition can be applied withconventional hand or internally powered spraying equipment. In thismanner, the coating composition can be readily applied to mountedresistors, electronic circuit boxes, and similar devices.

In the following illustrative examples all parts and percentages are byWeight unless otherwise stated.

EXAMPLE 1 A liquid-coating composition was prepared by mixing a blushlacquer solution with a large number of tiny half-reflectorized glassbeads as follows: The blush lacquer solution was prepared by dissolvingi8 parts by weight of polystyrene resin in 66 parts by Weightdichloromethane and mixing the solution with 18 parts by weight of ethylalcohol. The glass beads, having a refractive index ofat least L8 anddiameters in the range of l0-50 microns, had been previouslyreflectorized by vapor deposition of aluminum over one hemisphere. Avolume of glass beads approximately equal to 8 percent of the volume ofthe blush lacquer solution as prepared above was dispersed therein. Thiscomposition was coated on a releasable surface and allowed to dry underblushing conditions to give a selfsupporting blush film. The blush film,which had no initial retroreflective properties, became permanentlytransparent upon heating at a temperature of C. and thereafter gave abrilliant retroreflective response to a light beam directed toward itssurface.

EXAMPLE 2 A blush lacquer solution was prepared by dissolving 20 partsby weight low-viscosity ethyl cellulose in 20 parts by weight acetoneand mixing the solution with 60 parts by weight distilled water. Avolume of glass beads (as described in Example ll) equal to 50 percentof the volume of the blush lacquer solution was dispersed into themixture. This composition was cast into a film which was found totransparentize and become retroreflective after heating at C.

EXAMPLE. 3

A 10 weight percent solution of hexachloroethane in acetone wassubstituted for the acetone in a formulation otherwise similar toExample 2 to prepare an initially nonretroretlective film which uponbeing heated at C. for l2 minutes acquired retroreflective properties.

EXAMPLE 4 12.5 parts by weight polyethylene terephthalate resin, 12.5parts by weight distilled water and 75 parts by weight trifluoroaceticacid was combined and stirred to produce a blush lacquer solution. Avolume of glass beads (as described in Example 1) equal to about percentof the volume of the solution was dispersed therein. The composition,when coated and allowed to dry, produced a film which became transparentand retroreflective at 220 C.

EXAMPLE 5 A coating composition prepared according to Example A wasapplied as a paint to deposit a band around each of a number ofresistors in an electrical circuit contained within a box. A typicalexample is shown in FIG. 4. After the paint had dried, a light beamdirected into the box revealed no retroreflective response from themarked resistors. Excess current was allowed to pass through some of themarked resistors, intentionally causing them to overheat. The markedoverheated resistors gave a brilliant response of retroreflected lightto a light beam directed into the box, making it a very simple matter toisolate those resistors which had overheated. No retroreflectiveresponse was seen in the marked resistors which were not subjected toexcess current.

What is claimed is:

1. A liquid coating composition comprising a mixture of blushfusible-lacquer solution and particles adapted to retroreflect light,ofa consistency adapted for application to a base surface to provide athin uniform coating thereon consisting ofa dried heat-sensitive blushfusible lacquer layer obscuring an imbedded layer of said particles,said coating becoming visibly retroreflecting upon simple heating.

2. The coating composition of claim ll wherein said particles are tinytransparent spheres having an integral specularly reflective metalcoating on one hemisphere.

3. The coating composition of claim 2 including a volatile propellentliquid.

4 An article having a surface which becomes retroreflective upon heatingand comprising particles adapted to retroreflect light imbedded within asolid fusible obscuring layer which achieves a fluid state duringheating to permit said fluid to be displaced physically exposing atleast a portion of the surfaces of said beads.

5. The article of claim 4 wherein said retroreflective particles includeheat-resistant transparent heads.

6. The article of claim 4 wherein said heat-sensitive obscuring layerconsists of blush lacquer.

7. The article of claim 4 further comprising a substrate having attachedto one surface thereof said retroreflective particles.

8. The article of claim 7 wherein the substrate is an electrical circuitcomponent.

2. The coating composition of claim 1 wherein said particles are tinytransparent spheres having an integral specularly reflective metalcoating on one hemisphere.
 3. The coating composition of claim 2including a volatile propellent liquid.
 4. An article having a surfacewhich becomes retroreflective upon heating and comprising particlesadapted to retroreflect light imbedded within a solid fusible obscuringlayer which achieves a fluid state during heating to permit said fluidto be displaced physically exposing at least a portion of the surfacesof said beads.
 5. The article of claim 4 wherein said retroreflectiveparticles include heat-resistant transparent beads.
 6. The article ofclaim 4 wherein said heat-sensitive obscuring layer consists of blushlacquer.
 7. The article of claim 4 further comprising a substrate havingattached to one surface thereof said retroreflective particles.
 8. Thearticle of claim 7 wherein the substrate is an electrical circuitcomponent.